The present invention concerns a device for supplying a finely divided product, such as for example, a coating product in powder form, to an installation for spraying the product.
For many industrial purposes, the finely divided product, commonly called "powder", is delivered from a reservoir to its point of utilization such as, for example, a sprayer of the pneumatic or centrifugal type. The powder is conveyed with the aid of a jet of air along a conduit having a substantial length, which conduit often follows a tortuous path and has a relatively small diameter.
It is essential that the supply of powder be continuous, homogenous and stable during a prolonged period and that the supply be controllable with precision and rapidity. For this purpose, one can utilize a suction device of the Venturi type. Use can also be made of a worm screw or an Archimedes screw mounted at the bottom of a hopper and ending at a pipe supplied with entrainment air by an injector. Such a system presents advantages over powder suction devices, notably because of the relative regularity of the mass flow of powder in spite of variable losses in load. The present invention concerns an improvement in systems of this type.
The powders utilized are generally intended to be baked at around 180.degree. C. after having been applied. Now, it has been noted that this baking phase causes yellowing of certain powders. Powders which have been recently developed can be baked at only around 140.degree. C. This permits resolution of the problem of yellowing but introduces another limitation because these powders experience rapid alteration at ambient temperature. They must be stored at low temperatures, preferably between 5.degree. C. and 10.degree. C. In addition, the polymerization point of these powders is low and they are liable to polymerize at points in the supply system where the temperature has a tendency to be elevated, such as, for example, around the worm screw of a mechanical entrainment system, or in the sheath, or tube, surrounding this endless screw.
One of the essential advantages of coating with a finely divided product is that the product which has not struck the target, i.e. the object to be painted, can be recycled by an appropriate recovery device for the powder in the spray enclosure and a device for transporting this recovered powder back to the device for supplying powder to the spraying installation, where the recovered powder is mixed with new powder. The powder undergoes heating during spraying. If the temperature at the outlet of the supply device is too high, there is thus a risk that the powder is not reusable. In addition, there is a risk that the powder which is recovered will raise the temperature of the new powder during mixing of the two, even if the new powder has been stored under refrigeration until the moment when it is poured into the reservoir of the supply system.
The torque resulting from rubbing between the worm screw and the powder, on the one hand, and between the tube and the powder on the other hand, is for the most part dissipated in the form of heat, which has a tendency to raise the temperature of the powder and, as a consequence, to deteriorate the powder.
It has been envisioned to cool the powder during its travel in the supply system or between this system and the associated sprayer. However, the powder is generally made from organic resins, for example epoxy, polyester, acrylic, polyurethane, etc. These materials are poor thermal conductors which are difficult to cool. Raising the temperature of the powder thus creates a situation which it is difficult to correct. In addition, in systems of the prior art, the energy transmitted to the worm screw is susceptible to uncontrolled variations. If it increases, there can be dangerous heating of the powder. If it diminishes, the supply of powder may not be sufficient. In addition, phenomena of depriming (i.e. emptying of the conveying path due to lack of powder or excessive fluidization of the powder) or the formation of "plugs" of powder around the worm screw can take place and these systems must be monitored continuously by an operator to avoid these occurrences.